Multi Power Source Systems for Photovoltaic Battery Control

ABSTRACT

A multi-power source system including a first power source, a second power source in a parallel with the first power source, and a diode preventing power from the second power source to drive the first power source but permitting the first power source to charge the second power source. The system also includes a controller operably coupled to both the first and second power sources, and a plurality of field effect transistor (FETs) arranged in series with one or more of the first power source, the second power source, and the load, wherein controller can switch the plurality of FETs to enable the first power source to drive the load or the second power source to drive the load.

TECHNICAL FIELD

The present disclosure relates to dual power systems and controlalgorithms for determining which to apply to a load. More particularly,the present disclosure is directed to solar systems, and moreparticularly self-powered solar tracking systems and the control systemsand algorithms for switching between solar power and battery power todrive the solar trackers.

BACKGROUND

There have been developed a number of solutions power source control indual and multi-source power systems. In the solar tracker scenario, andparticularly the self-powered solar tracker scenario, as described incommonly owned U.S. Patent Publication No. 2016/0308488 filed Dec. 15,2016, and entitled Self Powered Solar Tracker Apparatus, there have beendeveloped certain control systems. One of these control systemsdetermines the source of the power to be applied to a drive motor whichdrives the solar tracker, following the sun, in order to ensure thatsolar panels are positioned for maximum energy production. One sourcethat can be used is the power generated by a solar module. Typically,this solar module is specifically assigned only for generation of powerto drive the motor. A single panel, even a relatively small panel, isoften sufficient to drive the motor, which may only require about 15 Wper day (generally between about 10 W and 25 W per day) to drive thesolar tracker. In part this very small load is a testament to thebalancing of the solar trackers themselves and the high precisionengineering which has significantly reduced the mechanical load throughbalancing and reduction of friction within the system.

Despite the relatively low load of the system, there remain times whenthe energy produced by the dedicated solar cell is insufficient to drivethe motor. This may occur when, for example, the systems are returningto a morning start position following the setting of the sun. Or it mayoccur when the sun is obscured by clouds and the solar panel is notgenerated sufficient power to drive the motor. In these types ofinstances, a battery is employed to drive the solar tracker. As will beappreciated, the ability to switch between the two power providingsystems (i.e., the solar panel or the battery) is an important featureof any such system. Though there have been developed systems enablingthis transition, there is always a need for improved and more efficientsystems.

SUMMARY

The present disclosure is directed to a multi-power source systemincluding a first power source, a second power source in a parallel withthe first power source, and a diode preventing power from the secondpower source to drive the first power source, but permitting the firstpower source to charge the second power source. The system furtherincludes a controller operably coupled to both the first and secondpower sources, and a plurality of field effect transistor (FETs)arranged in series with one or more of the first power source, thesecond power source, and the load, wherein controller can switch theplurality of FETs to enable the first power source to drive the load orthe second power source to drive the load.

The first power source may be an array of solar panels, for example asolar tracker comprised of a plurality of solar panels. The load may bea drive motor for driving the solar tracker. The second power source maybe a battery.

The system may further include a plurality of proportion, integral,derivative controllers to compare the output of the first and the secondpower sources. Further, the plurality of FETs may be two FETs whichoperate in opposing manners. In accordance with one aspect of thedisclosure, the system further includes an inductor in series with thesecond power source, wherein the FETs are configured to charge thesecond power source by the first power source by controlling thedirection of a current across the inductor to be a negative magnitude.Additionally or alternatively, the system further includes an inductorin series with the second power source, wherein the FETs are configuredto cause the first power source to supply power to the load bycontrolling the direction of a current across the inductor to be apositive magnitude. Still further, the system includes an inductor inseries with the second power source, wherein the second power source iselectrically disconnected from the load by controlling the direction ofa current across the inductor to be zero.

Additionally or alternatively, the system may further include acapacitor that is in parallel with the load motor. Further when the busvoltage is lower than a low voltage threshold, the plurality of FETs aremodulated to charge the second power source by the first power source.Still further, when the bus voltage is higher than a high voltagethreshold, the plurality of FETs are disabled. Further, when the busvoltage is between a high voltage threshold and a low voltage threshold,a charge status is checked on the second power source, in a case thatthe charge status indicates that the second power source is low then theplurality of FETs are modulated to charge the second power source by thefirst power source.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the present disclosure are described herein belowwith reference to the drawings, which are incorporated in and constitutea part of this specification, wherein:

FIG. 1 depicts a self-powered solar tracker systems in accordance withthe present disclosure;

FIG. 2 depicts a detailed view of a drive mechanism of a self-poweredsolar tracker in accordance with the present disclosure;

FIG. 3 depicts a schematic of a control system for a self-powered solartracker in accordance with the present disclosure;

FIG. 4 depicts a schematic of a multi-source power systems in accordancewith the present disclosure;

FIG. 5 depicts a hardware schematic of FIG. 4 including details of abi-direction power control circuit in accordance with the presentdisclosure;

FIG. 6 depicts a control schematic associated with FIGS. 4 and 5 inaccordance with the present disclosure; and

FIG. 7 depicts a logic flow for a control algorithm in accordance withthe present disclosure.

DETAILED DESCRIPTION

The present disclosure is directed to systems and methods forcontrolling a dual power system whereby a single load may be driven bytwo separate power sources, both individually and together. Thoughdescribed generally herein in the context of a self-powered solartracking apparatus that utilizes both a photovoltaic (solar) panel and abattery to provide energy to drive a motor that rotates the trackerassembly, the systems, schematics, and algorithms described herein inany situation where there is are two power sources. In particular thesystems and algorithms of the present disclosure are useful where thereis one power source that is the preferred power source to be utilizedbut the system should experience little to no lag in transitioning tothe other power source. A further context for the present disclosure isin the area of a solar farm which is connected to a large power grid andmay be associated with large battery banks that can be used to providepower to the grid when the solar panels are unable to meet demand.Commonly owned U.S. Pat. Pub. 2017/0288184 entitled “Standard energystorage container platform,” filed Mar. 31, 2017 and teaches a batterycontainer and U.S. patent application Ser. No. 15/872,071 entitled“Direct Current Battery String Aggregator for Standard Energy StorageEnclosure Platform,” teaches a controller and system for connecting abattery and photovoltaic system to an energy grid. Both references areincorporated herein by reference. Other dual power source energy systemsrequiring monitoring and switching between energy supply systems arealso contemplated within the scope of the present disclosure.

FIG. 1 depicts a solar tracker system 10 which is commonly deployed aspart of a larger array. Each tracker 10 includes a plurality ofphotovoltaic panels 12 (solar panels). A motor 14 drives a shaft 15, towhich the solar panels 12 are affixed. By driving the shaft 14, thesolar panels 12 are maintained at a proper angle to the sun to ensuremaximum electrical power generation. The shaft 15 is suspended betweenthe motor 14 and a swinging or rotating mount 16. Both the motor 14 andthe rotating mounts 16 are supported on posts 18.

FIG. 2 depicts the area of the tracker system 10 near the motor 14. Ascan be seen a dedicated drive solar panel 20 is located in proximity tothe motor 14 and supported by the shaft 15. Either suspended from theunderside of the shaft 15 or mounted to the post 18 is a box 22. The box22 houses a battery 24, for example a lithium ion (Li-ion) battery, anda controller 26. The controller 26 provides input to the motor 14regarding whether to drive and how far to drive the shaft 15 to enablethe panels 12 to track the sun.

An example of the controller 26 can be seen in FIG. 3. The controller 26includes a control region 28 which houses a communications module 30(e.g., Zigbee, Wi-fi, Bluetooth®, etc.), an inclinometer 32, and a maincontroller (MCU) 34. The main controller 34 communicates with a batterycharger 36 to control the charging of the batteries 24, and with a motordrive controller 40, which controls the driving of the motor 14. Asdepicted in FIG. 3, the solar panel 20 provides electricity to thebattery charger 36, which at the discretion of the main controller 34 iseither directed to the battery 24 for charging or to a boost converter38 for application to the motor 14 to actually cause the motor 14 to bedriven. The main controller 34 can also determine, based on the inputfrom the solar panel 20, whether the energy being supplied isinsufficient to drive the motor 14, and can cause the stored energy inthe batter 24 to be utilized for this purpose.

FIG. 4 is a high level schematic of dual source power supply inaccordance with the present disclosure that may be used in place of orin conjunction with the components of FIG. 3. The solar panel 20 is inparallel with the battery 24 across a central bus 42. A load (e.g.,motor 14) is also connected in parallel with the bus 42. A controller(e.g., main controller 34) receives inputs from each of the solar panel20, the battery 24 and the bus 42. Based on these inputs the maincontroller 34 may determine whether to drive the motor 14 using theoutput of the solar panel 20, the output of the battery 24, and in whatproportion to apply each. Still further, the controller may determinewhen to charge the battery 24. The goal is to be able to use either thesolar panel 20 or the battery 24 without disrupting the driving of themotor 14. The signals from the main controller 34 are input to abi-directional power converter 44 to achieve the desired output from thesolar panel 20, battery 24, or both, or charging of the battery 24, aswill be described in greater detail below.

FIG. 5 is a hardware schematic of the system depicted in FIG. 4. Thesolar panels 20 provide an output to the central bus 42 (represented bythe capacitor C1). A pair of field effect transistors (FET) 46 and 48are utilized to selectively allow current to flow to and from thebattery 24, or to prevent current flow from the battery 24. The solarpanel 20 is always supplying whatever current it is generating to thecentral bus 42 and therewith the motor 14. The pair of FETs 46 and 48open or close to regulate the voltage charging the battery 24,discharging the battery 24, or removing the battery 24 from the circuit,based on a determination by the main controller 34. The pair of FETs 46and 48 are turned on or off, (e.g., pulsed) at a rate of, for example,50 KHz to accomplish these three states.

When it is determined that the battery is sufficiently charged, and thesolar panel 20 is providing sufficient power to drive the motor 14, thecontroller 34 will control the average duty cycle of the pair of FETs 46and 48, which are pulsed, such that the solar panel 20 is predominatelyproviding power to the motor 14 and providing limited charging of thebattery 24, as appropriate to maintain full charge of the battery 24. Inthis way, charge and discharge cycling of the battery can be minimizedand the life expectancy of the battery improved. Specifically, thebattery 24 is not being constantly charged from the solar panel 20, andis only being discharged when it is determined that the solar panel 20is not providing sufficient power (current) to drive the motor 14. Ifthe battery 24 is charged and the solar panel is providing sufficientpower then the battery 24 is essentially removed from the dischargecircuit to prevent inadvertent draw down of its power.

FIG. 6 is a control schematic depicting the logic required to providethe input to the FETs 46 and 48 and control charge, discharge, andremoval of the battery 24. In FIG. 6, the voltage output by the solarpanel 20, for example determined using maximum power point tracking(MPPT) is compared to a reference voltage supplied by the maincontroller to determine which is greater in Min/Max 51. The output ofthat Min/Max 51 is then compared in comparator 52 to a referencevoltage. The output of the comparator 52 is input to a PID(proportional, integral, derivative) controller 54. The output of thePID controller is then supplied to a Min/Max 56.

Simultaneously, with the solar panel 20 output determinations describedabove, a similar determination is made with respect to the battery 24.The battery 24 voltage is compared to a reference in comparator 58. Theoutput of the comparator 58 is passed through a second PID controller60. The output of the comparator 58 is also supplied to the Min/Max 56.Min/Max 56 compares the output of the PID controller 60 to the output ofthe PID controller 54, where the larger value is provided as an input tocomparator 62. This value is then compared by comparator 62 to battery24 current. Battery 24 current is measured at inductor 25. A positivecurrent at the inductor 25 indicates that the battery 24 is discharging,and a negative current means that the battery 24 is charging. The resultfrom comparator 62 value is fed into PID controller 64 to drive the pairof FETs 46 and 48 such that the battery 24 is charging, discharging, orremoved from the circuit as appropriate to properly maintain the battery24.

Instead of monitoring the output of the solar panel 20 and the battery24, a second algorithm, depicted in FIG. 7, can be employed to determinethe status of FETs 46 and 48. As depicted in FIG. 7 the voltage of thebus 42 is monitored. With reference to FIG. 5, the bus 42 voltage is thevoltage across capacitor C1.

In the outer loop of the diagram, the solar panel 20 and the battery 24provide sufficient voltage across the bus 42 (C1). The bus 42 voltage isconstantly monitored 71 by the controller 34. If the bus 42 voltagemeasures below a low voltage threshold voltage (for example, lower thanthe MPPT (maximum power point voltage) setting the bus 42 voltage isthen regulated 72 by the pair of FETs 46 and 48. The first FET 46 andthe second FET 48 are pulsed by the controller 34 to the MPPT voltageat, for example, a rate of 50 KHz. If the bus 42 voltage measures higherthan the high voltage threshold, the pair of FETs 46 and 48 are disabled74 and the controller 34 checks to see if the bus 42 voltage is betweenthe low bus voltage and the battery 24 voltage 74. If the bus 42 voltageis not between the low bus voltage and the battery 24 voltage 74, thenthe pair of FETs 46 and 48 are disabled 74 and the controller 34 goesback to monitor mode 71. If the bus 42 voltage is between the low busvoltage threshold and the battery 24 voltage then the FETS are enabledto regulate 72 the bus 42 voltage. If the bus 42 voltage is in betweenthe high voltage threshold and the low voltage threshold then thecontroller 34 checks to see if the battery 24 is fully charged 73. Ifthe battery 24 is not fully charged, then the controller 34 enables thepair of FETs 46 and 48 to regulate the bus 42 voltage 72 at MPPTvoltage. If the battery 24 is fully charged then the pair of FETs 46 and48 are disabled 74 and the controller 34 goes back to monitor mode 71.In this fashion, the bus 42 voltage can be kept relatively constant.Which means the PV energy, battery energy, and load demand are in abalanced situation. Further, cycling of the battery 24 between chargingand discharging can be minimized once the battery 24 is fully chargedand the solar panels 20 are providing sufficient voltage across bus 42.This charging and discharging is controlled by changing the average dutycycle of the first FET 46 and second FET 48. Further, the battery 24 maybe periodically checked both during charging and when not charging toensure that it is ready and able to meet demand of the motor 14 whenneeded.

While several embodiments of the disclosure have been shown in thedrawings, it is not intended that the disclosure be limited thereto, asit is intended that the disclosure be as broad in scope as the art willallow and that the specification be read likewise. Any combination ofthe above embodiments is also envisioned and is within the scope of theappended claims. Therefore, the above description should not beconstrued as limiting, but merely as exemplifications of particularembodiments. Those skilled in the art will envision other modificationswithin the scope of the claims appended hereto.

We claim:
 1. A multi-power source system comprising: a first powersource; a second power source in a parallel with the first power source;a diode preventing power from the second power source to drive the firstpower source, but permitting the first power source to charge the secondpower source; a controller operably coupled to both the first and secondpower sources; and a plurality of field effect transistor (FETs)arranged in series with one or more of the first power source, thesecond power source, and the load, wherein controller can switch theplurality of FETs to enable the first power source to drive the load orthe second power source to drive the load.
 2. The multi-power sourcesystem of claim 1 system wherein the first power source is an array ofsolar panels.
 3. The multi-power source system of claim 2 system whereinthe first power source is a solar tracker comprised of a plurality ofsolar panels.
 4. The multi-power source system of claim 3, wherein theload is a drive motor for driving a solar tracker.
 5. The multi-powersource system of claim 1, wherein the second power source is a battery.6. The multi-power source system of claim 1, further comprising aplurality of proportion, integral, derivative controllers to compare theoutput of the first and the second power sources.
 7. The multi-powersource system of claim 1, wherein the plurality of FETs comprises twoFETs which operate in opposing manners.
 8. The multi-power source systemof claim 7, further comprising an inductor in series with the secondpower source, wherein the FETs are configured to charge the second powersource by the first power source by controlling the direction of acurrent across the inductor to be a negative magnitude.
 9. Themulti-power source system of claim 7, further comprising an inductor inseries with the second power source, wherein the FETs are configured tocause the first power source to supply power to the load by controllingthe direction of a current across the inductor to be a positivemagnitude.
 10. The multi-power source system of claim 7, furthercomprising an inductor in series with the second power source, whereinthe second power source is electrically disconnected from the load bycontrolling the direction of a current across the inductor to be zero.11. The multi-power source system of claim 7, further comprising acapacitor is in parallel with the load motor, wherein a bus voltageacross the capacitor is monitored.
 12. The multi-power source system ofclaim 11, wherein when the bus voltage is lower than a low voltagethreshold, the plurality of FETs are modulated to charge the secondpower source by the first power source.
 13. The multi-power sourcesystem of claim 11, wherein when the bus voltage is higher than a highvoltage threshold, the plurality of FETs are disabled.
 14. Themulti-power source system of claim 11, wherein when the bus voltage isbetween a high voltage threshold and a low voltage threshold, a chargestatus is checked on the second power source, in a case that the chargestatus indicates that the second power source is low then the pluralityof FETs are modulated to charge the second power source by the firstpower source.